Radiation sources in charged particle accelerators



M. H. HEBB Aug. 20, 1 957 RADIATION SOURCES IN CHARGED PARTICLEACCELERATORS Filed Sept. 50, 1952 Inventor": Malcolm 1'1. Hebb,

His AttOTneH.

United States Patent RADIATIQN SOURCES IN CHARGED PARTICLE ACCELERATORSMalcolm H. Hebb, Schenectady, N. Y., assignor to General ElectricCompany, a corporation of New York Application September 30, 1952,Serial No. 312,260 Claims. (Cl. 31362) The present invention relates tocharged particle accelerator apparatus and, more particularly, toradiation sources in charged particle accelerator apparatus.

Apparatus for accelerating charged particles by means of magneticinduction effects is .shown and described in United States Patent Nos.2,394,071, 2,394,072 and 2,394,073, all of which were patented February5, 1946 by Willem F. Westendorp and assigned to the assignee of thepresent invention. Such apparatus can comprise a core of magneticmaterial including a pair of opposed, rotationally symmetrical polepieces which define a toroidal gap wherein an evacuated container ispositioned. The core is excited by means of windings that are energizedby a source of time-varying voltage to produce a time-varying magneticfiux which links an equilibrium orbit within the evacuated container anda time-varying magnetic guide field which traverses the equilibriumorbit. Charged particles, e. g. electrons injectedalong the equilibriumorbit from'an electron gun positioned adjacent to the orbit within theregion of influence of the time-varying magnetic guide field, areaccelerated to high energy levels,

by the time-varying magnetic flux during a great number of revolutionswhile the time-varying magnetic guide field constrains the particles tofollow paths along the equilibrium orbit. After acceleration to adesired energy level, the charged particles can be diverted from theequilibrium orbit to a target for the generation of X-radiation.

A major problem in the utilization of magnetic induction acceleratorapparatus and other forms of accelerator apparatus employing atime-varying magnetic guide field is that of obtaining so-called thicktarget X-radiation. Thick target radiation is that obtained from atarget having a thickness in the direction of travel of impingingparticles sufficientto stop substantially all ofthe particles and thusto convert a maximum fraction of the particle energy into radiation.- Indiverting the charged particles from theequilibrium orbit of acceleratorapparatus of the above-described forms, the particles are caused tospiralawayfrom the equilibrium orbit until their paths intersect thetarget. Since these paths are necessarily slow spirals, i. e. of smallpitch, the particles succeed only in striking the leading edge of athick target; whereby many particles are lost by scattering through theedges of the target without having a maximum fraction of their energyconverted to X'radiation as desired. I

it istherefore a principal object of the present inventiontoprovide anefiicient means of obtaining thick target radiation fromcharged-particle accelerator apparatus of the forms described.

According to one aspect ofthe invention, a thin target, i. -e. a targetwhichcharged particlescan traverse without an appreciable loss inenergy, is positioned adjacent to the equilibrium orbit in azimuthallyspaced relationship with respect to a thick: target in charged particleaccelerator apparatus. Bothof the targets are located on the same sideof, the equilibrium orbit. The relative positions of thetwo targetspermit the particles diverted fromthe 2,803,766 Patented Aug. 20, 1957ICQ equilibrium orbit first to strike and be scattered by the thintarget and subsequently to strike the thick target well within its edge,whereby thick target radiation is efii ciently generated.

The features of the invention desired to be protected herein are setforth in the appended claims. The invention itself, together withfurther objects and advantages thereof, may best be understood byreference to the fol- Fig. 3 is a section view taken along lines 3--3 ofFig. 2; and

Fig. 4 is a section view taken along lines 44 of Fig. 2 Referringparticularly now to Fig. 1, there is shown in exemplary fashion magneticinduction accelerator apparatus suitably embodying the invention. Theapparatus comprises a magnetic core 1 which can be laminated to minimizethe generation of eddy currents therein. Core 1 includes laminated,rotationally symmetrical, opposed pole pieces Zland 3 having generallyoutwardly tapered pole faces 4 and 5 for the provision of a magneticguide field traversing an equilibrium orbit O, as will be more fullydescribed hereinafter. Coaxial with pole pieces 2,

3 and disposed between pole faces 4, 5 is an evacuated annular containeror envelope 6 of dielectric material,

which provides within its interior an annular chamber 7 wherein chargedparticles can be accelerated. The central portions of pole pieces 2, 3are terminated respectively by fiat, surfaces 8, 9 between which aredisposed laminated metallic disks (not shown) and dielectric supportspacers 1t 11. The metallic disks serve the purpose of reducing thereluctance of the magnetic path in the region between surfaces 8 and 9.

Magnetic core 1 can be excited from a suitable source of time-varyingvoltage 12 connected as: indicated to series-connected energizingwindings 14, 1S surrounding pole pieces 2, 3. To minimizethe currentdrawn from source 12, energizing windings 14 and 15 can be resonated bypoWer-factoncorrecting capacitors 16. Within chamber 7 adjacent toequilibrium orbit O and also within the region of influenced thetime-varying magnetic guide field existing between pole faces 4, 5during operation of the apparatus, there is provided a charged particlesource 17 which is supported from a hermetically-sealed side arm 18 ofenvelope 6. More detailed illustration and description of electron gunstructure suitable for present purposes can be found by reference to theabove-mentionedpatents orby reference to the United States Patent No.2,484,549 of J. P. Blewett, patented October 11, 1949 and assigned tothe assignee of the present invention.

It is well understood by those familiar with magnetic inductionaccelerator apparatus that energization of windings 14, 15 by the sourceof time-varying voltage 12 results in a time-varying magnetic flux whichtraverses magnetic core 1 and pole pieces .2, 3 to provide a timevaryingmagnetic flux that links equilibrium orbit O and a time-varying magneticguide field that traverses the locus of equilibrium orbit 0 and thevicinity thereof between pole faces 4, 5. Electrons emitted by gun 17 ata desiredtimed instant near zero in the cycle of magnetic flux and fieldvariations are continuously ac-.

celerated during the acceleration portion of the cycle as they executerepeated revolutions along and about equi librium orbit 0. As aconsequence, the injected electrons can be caused to assume energies ofmany millions ofelectron volts. and then can be automaticallydivertedfrom the equilibrium orbit by means of pulsatingly energized orbit shiftcoils 19, 20 to produce X-radiation in a manner which will be more fullydescribed hereinafter. Means including circuits for arranging the propertimed injection and subsequent diversion of the charged particles fromthe equilibrium orbit at or near the end of the acceleration cycle aredisclosed in the aforementioned patents and additionally in the UnitedStates Patent No. 2,394,070 of D. W. Kerst, patented February 5, 1946and assigned to the assignee of the present invention. As has beenexplained in D. W. Kerst Patent No. 2,297,305, patented September 29,1942 and assigned to the assignee of the present invention, thetime-varying magnetic flux linking the equilibrium orbit may be causedto produce centripetal forces which balance the centrifugal forces uponthe charged particles undergoing acceleration at all times throughoutthe acceleration cycle, providing the following relationship issatified:

where A is the total change in flux linking the equihbrium orbit, R isthe radius of the orbit and B0 is the flux density of the time-varyingmagnetic guide field at the equilibrium orbit. The condition specifiedby this relationship may be realized by making the reluctance for oneunit area of cross section of the magnetic path of the time-varying fiuxgreater by an appropriate amount at the equilibrium orbit than itsaverage reluctance for one unit area of cross section within the orbit.

The fulfillment of the foregoing condition, however,

only assures stable acceleration for those charged particles which areinjected tangentially to their instantaneous circles or orbits. Theinstantaneous circle or orbit is the circular orbit along which acharged particle started at the proper position with the right energywill travel in a time-constant, radially symmetric magnetic field. Witha time-varying magnetic flux as above specified, the loci of theinstantaneous circles of all the charged particles approach andeventually essentially coincide with the equilibrium orbit during thelatter portions of the acceleration cycles. Consequently, meeting theforegoing condition does not take into consideration the requirementsfor stable acceleration of charged particles which tend for one reasonor another to deviate from their respective instantaneous circles or todeviate from the equilibrium orbit when their respective instantaneouscircles coincide therewith. Nevertheless, by arranging the spatialdistribution of the time-varying magnetic guide field in the vicinity ofthe equilibrium orbit as specified by the following relationship, bothradial and axial focusing forces which tend to constrain deviatingparticles to their respective instantaneous circles or to theequilibrium orbit can be provided:

where H is the intensity of the time-varying magnetic guide field in thevicinity of the equilibrium orbit, r is the radius of a particular pointunder consideration and n is a parameter having a value lying betweenzero'and one. The outwardly directed taper of pole faces 4 and 5 asillustrated in Fig. 1 enables the utilization of the condition set forthin Equation 2. It is apparent from Equation 2 that the parameter n is ameasure of the rate of decrease of the time-varying magnetic guide fieldwith radius. Both radial and axial focusing forces exist if 0 n 1. For auniform field, n=0 and no axial focusing of the particles can takeplace. In a field inversely proportional to the radius (11:1), there areno radial focusing forces. Since both radial and axial focusing forcesare required to secure collimation of the particle beam duringacceleration, the foregoing limits are placed upon the selected value ofthe n.

After the charged particles have been accelerated to a desired energylevel under the foregoing conditions, they must be diverted from theequilibrium orbit to permit useful utilization of the energy which hasbeen imparted to them. As has been stated above, diversion of thecharged particles from the equilibrium orbit can be accomplished bysupplying a properly timed pulse of current to orbit shift coils 19 and20. The application of the current pulse to the orbit shift coilsmodifies the magnetic induction throughout the stable region surroundingequilibrium orbit O and causes the charged particles to spiral veryslowly inwardly or outwardly from the equilibrium orbit, depending upon,the direction of the flux generated by the orbit shift coils withrespect to the flux generated by windings 14, 15. During their travelaway from the equilibrium orbit the charged particles can be consideredas following paths tangent to their respective instantaneous circles,the radii of which are gradually decreasing or increasing as the casemay be. Since the charged particles do follow spiral paths of such smallpitch, it is readily understood that the positioning of a thick X-raygenerating target adjacent to the equilibrium orbit does not provide anopportunity for efiiciently generating thick target radiation.Essentially all of the charged particles diverted from the equilibriumorbit in this manner will strike the leading edge of the thick target,whereby many of the particles will be scattered through the edges of thetarget without a maximum fraction of their energy having been convertedinto X-radiation.

Accordingto the present invention thick target X- radiation may beefficiently obtained from charged particle accelerator apparatus byfirst causing the charged particles to be scattered by a thin target andsubsequently causing the charged particles to strike a thick target.Referring specifically now to Figs. 2, 3 and 4, there is shown a thintarget 21 and a thick target 22, both of which are positioned adjacentto and on the same side of equilibrium orbit 0. Thin target 21, whichmay comprise a thin strip of a low atomic number material such asberyllium, aluminum, magnesium, etc., is supported by a bent rigid rod23 that is hermetically sealed into the wall of envelope 6 asillustrated. Rod 23 is extended downwardly shortly after it enterschamber 7 in order that charged particles will not impinge thereuponduring their acceleration along equilibrium orbit 0. Thick target 22.which may comprise a relatively thick plate of a heavy material such astungsten, molybdenum, copper, etc, is adjustably supported from a rigidrod 24 which is introduced into chamber 7 through a hermetically sealedbellows 25. The portion of rod 24 extending transversely of envelope 6is likewise removed from the vicinity of equilibrium orbit O in order toavoid premature collision of the charged particles thereupon. The outeredge of thick target 22 is positioned more radially inward than theouter edge of thin target 21 so that charged particles spiralinginwardly from equilibrium orbit O first strike thin target 21.Preferably, the inner edge of thin target 21 is placed at essentiallythe same as or at a larger radius than the outer edge of thick target 22when the charged particles are shifted inwardly from equilibrium orbit Oby coils 19 and 20.

Now it will be understood that when orbit shift coils 19, 20 areenergized with a pulse of current to cause the charged particles tospiral inwardly after they have been accelerated to a desired energylevel as described in the aforementioned Kerst Patent No. 2,394,070, thecharged particles first strike thin target 21. According to theinvention, thin target 21 is so selected that it produces primarily ascattering of the charged particles which impinge upon it. In thismanner the trajectories of the charged particles are changed abruptly,thus facilitating the generation of thick target radiation by thesubsequent impingement of the scattered particles upon thick target 22.Thick target 22 is positioned farther from equilibrium orbit O and at adefined azimuthal spacing with respect to thin target 21 in order that amaxium number fields in the target.

saga-tree The considerations involved the selection .of the properposition of thick target 22 with respect tothin target 21 will bediscussed presently.

When charged particles such as fast electrons pass through target foilssuch asthin target 21, several processes occur. First, some'of theenergy of the charged particles is converted to radiation such as;X-radiation. This process chiefly involves interaction with the nuclearfield of the target atoms and can be determined by the followingrelation:

where Wr is the quantity of energy radiated in millions of,

where W1 is the energy lost by ionization in millions of electron volts.Thirdly, the charged particles striking the target element are multiplyscattered. This process is the charged particle deflection accumulatedas the result of many successive collisions, mainly with the nuclear Ittransforms an initially collimated beam into one with a Gaussiandistribution in space, the root-mean-square angle of scattering beingt.,.=-.-, N As is explained above, the present invention contemplatesthe utilization of the scattering phenomenon in thin target 21 to changeabruptly the trajectoriesof the charged particles impinging thereupon,whereby the particles can subsequently strike a thick target to generateradiation by the radiation process. The above equations show that thescattering process decreases with higher energies of the chargedparticles hence it is preferable that the charged particles not beaccelerated to an excessively high energy. This effect is illustrated bythe ratio of Equation 3 to Equation 5 which indicates that an adequatescattering target will generate a considerable amount of X- radiation athigh energies and therefore will itself become a fairly thick target dueto multiple passages of. the particles, however the energy at which thisbecomes obje'ctionable can be made higher by choosing a low atomicnumber for target 21. Since ionization loss in target 21 is undesirablein the present invention, it will be seen from Equations 4 and 5 that aminimum thickness should be selected for target 21.

After the charged particles have struck thin scattering target 21 andhave passed therethrough, their trajectories are of course affected bythe restoring forces of the abovedescribed time-varying magnetic guidefield. Essentially none of the charged particles will be on itsrespective instantaneous circle, hence each will oscillate about its owninstantaneous circle. The path followed by each particle about its owninstantaneous circle has a radial fre quency equal to /1nf and an axialfrequency equal to V17 where f is the equilibrium orbital revolutionfrequency. According to the present invention, it is preferable thatthick target 22 be in azimuthally spaced relationship with respect tothin target 21 such that target 22,

will intercept the scattered charged particles at the position where thecharged particle beam is most Widely dispersed. With thick target 22placed in such a defined position it is possible to intercept nearlyone-half of the scattered charged particles upon their first revolution.Some of the remainder of the scattered charged particles pass throughthin target 21 upon subsequent revolutions and 7 are scattered, again.Many of these, re scattered particles;

strike thick target 22 thereafter, along withmany of the particles whichdid not strike target 22 upon their first revolution and were notre-scattered by target 21.

It has been found that the charged particle beam is radially most widelydispersed at a particular azimuthal angle with respect to thin target21. This angle is defined by the following relations:

where at is the azimuthal angle between the maximum dispersion of theparticles beam and thin target 21. Accordingly, when the chargedparticle beam is deflected radially from the equilibrium orbit,placement of thick target 22 approximately at an azimuthal angle 6;defined by Equation 6 produces maximum generation of thick targetradiation. The essentially separation of the targets illustrated in Fig.2 should be employed for an apparatus having a value of 0.75 for theparametern, as a substitution into Equation 6 shows.

It Will be understood that targets 21 and 22 can be positioned atgreater radii than equilibrium orbit O and orbit expansion utilized todirect the charged particles to thin target 21. In addition, the targetsmay be positioned axially above or below equilibrium orbit O With thintarget 21 located approximately at the radius of the equilibrium orbit.In this latter event the accelerated particle beam must be shiftedaxially as described in the aforementioned Westendorp Patent No.2,394,072; furthermore, since the frequency of the axial oscillations ofthe charged particles about their instantaneous circles differs from thefrequency of the radial oscillations, thick target 22 must be positionedat a difierent azimuthal angle with respect to thin target 21 in ordertosecure the advantageuof obtaining the most widely dispersed particlebeam. With axial orbit shift the azimuthal separation of the targets maybe determined approximately from the equation:

where 6.. is the azimuthal angle between the two targets,

From the foregoing description it is readily. appreciated that thepresent invention makes possible the generation of thick targetradiation with high efiicienoy in orbital charged particle acceleratorapparatus. Thick target radi; ation is very desirable in manyradiographic applications inasmuch as thick targets provide broaderX-ray beams with a greater total quantity of radiation than do thintargets. The present invention is not limited to utilization inconjuncion with accelerator apparatus which employs magnetic inductionphenomena alone, but can also. be used with synchrotron apparatus suchas that disclosed in United States Patent No. 2,485,409, patentedOctober 18, 1949 by Willem F. Westendorp and Herbert C. Pollock andassigned to the assignee of the present invention. Further applicationof the present invention can be made in connection withnon-ferromagnetic accelerating apparatus, e. g. the apparatus disclosedin United States Patent No. 2,465,786, patented March 29, 1949 by I. P.Blewett and assigned to the assignee of the present invention. Ingeneral, the present invention has application in all types of orbitalcharged particle accelerating apparatus having a time-varying magneticguide field essentially satisfying the condition of Equation 2.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A charged particle accelerating apparatus having an enclosuredefining a stable accelerating region containing an equilibrium orbitand traversed by a time-varying magnetic guide field, a thin targetpositioned within said enclosure and at a smaller radius than the radiusof the equilibrium orbit, and a thick target positioned Within saidenclosure with the outer edge of said. thick target at a smaller radiusthan the outer edge of said thin target but not substantially smallerthan the inner edge of said thin target so that charged particlesemerging from the 7 equilibrium orbit first strike said thin target andsubsequently strike said thick target.

2. A charged particle accelerating apparatus having an enclosuredefining a stable accelerating region containing an equilibrium orbitand traversed by a time-varying magnetic guidefield, a thin targetpositioned within said enclosure and at a smaller radius than the radiusof the equilibrium orbit with its nearest edge a predetermined distancefrom the equilibrium orbit, and a thick target positioned within theenclosure and in azimuthally spaced relationship with respect to saidthin target and on the same side of the equilibrium orbit with itsnearest edge approximately at the same radius as the farthest edge ofsaid thin target, whereby charged particles emerging from theequilibrium orbit first strike said thin target and subsequently strikesaid thick target.

- 3. A charged particle accelerator apparatus having an enclosuredefining a stable accelerating region containing an equilibrium orbitand traversed by a time-varying magnetic guide field essentiallysatisfying the relation d (log T) where H is the intensity of thetime-varying magnetic guide field within the stable accelerating region,r is the radius of a particular point under consideration and n is aparameter having a value lying between zero and one, a thin targetpositioned within the enclosure and at a smaller radius than the radiusof the equilibrium orbit, and a thick target positioned within theenclosure with the outer edge thereof at a smaller radius than the outeredge of said thin target and not substantially smaller than the radiusof the inner edge of said target, the azimuthal separation of saidtargets being approximately determined by the relation where 0r is theazimuthal angle between the two targets,

whereby charged particles diverted radially inwardly from theequilibrium orbit strike first said thin target and subsequently saidthick target.

d (log 1") Where H is the intensity of the time-varying magnetic guidefield within the stable accelerating region, r is the radius of aparticular point under consideration and n is a parameter having a valuelying between zero and one, a thin target positioned within theenclosure'adjacent the equilibrium orbit with its nearest edge apredetermined distance radially from the equilibrium orbit, and a thicktarget positioned adjacent the equilibrium orbit with its nearest edgeessentially at the same radius as the farthest edge of said thin target,the azimuthal separation of saidtargets being approximately determinedby the relation an equilibrium orbit and traversed by a time-varying magnetic guide field satisfying the relation d (log r) where H is theintensity of the time-varying magnetic guide field within the stableaccelerating region, r is the radius of a particular point underconsideration and n is a parameter having a value lying between zero andone, a thin target positioned within the enclosure adjacent theequilibrium orbit with its nearest edge a predetermined distance axiallyfrom the equilibrium orbit, and a thick target positioned within theenclosure adjacent the equilibrium orbit with its nearest edgeessentially at the same axial distance from the equilibrium orbit as thefarthest edge of said thin target, the azimuthal separation of saidtargets being approximately determined by the relation where 0., is theazimuthal angle between the two targets, whereby charged particlesdiverted axially from the equilibrium orbit strike first said thintarget and subsequently said thick target.

References Cited in the file of this patent UNITED STATES PATENTS

